rS485 shield for PI

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I am following the instructions on the wiki http://learn.linksprite.com/raspberry-pi/shield/user-tips-of-rs485-shield-for-rpi/ and am able to read data from my PC USB2RS485 device, however, attempts to write data results in dropped bits. The tx/rx lights on both the shield and my PC USB device light when the data is sent from the Pi, however, I'm unable to read the data correctly. I've tried several PC "terminal" type programs including reading the comm port directly via a program that I wrote.

I've tried writing via Python (the linksprite example), Java and minicom on the PI. All have the same result....reading is fine (data sent from the PC), however, data send back to the PC is not complete.

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Now it's difficult to tell which part is wrong. Can you use something like a TTL UART to USB convert to connect to your UART of Rpi, and to check if the UART is working at intended. if it does, then the RS485 shield is bad.